1. Field of the Invention
The present invention relates to an optical recording medium in which a dipyrromethene metal chelate compound is used and which is recordable and reproducible at a higher density as compared with a conventional case.
2. Description of the Related Art
Presently, as an optical recording medium having a larger capacity as compared with a CD, a digital video disc (DVD) , having a capacity of 4.7 GB has been developed and commercialized. The DVD is a ROM medium, and hence, recordable and reproducible optical record media having the corresponding capacity are desired. Among the DVDs, a write-once type is called a DVD-R.
In the DVD, in order to achieve a high-density recording, an oscillation wavelength of a laser beam is in the range of about 630 nm to 680 nm which are shorter wavelengths than in the case of the CD. As dyes for organic dye-including optical recording media which can be used within such short wavelengths, there have been suggested cyanine dyes, azo dyes, benzopyran-type dyes, benzodifuranone-type dyes, indigoid dyes, dioxazine-type dyes and porphyrin-type dyes in Japanese Patent Application Laid-open Nos. 74690/1992, 38878/1993, 40161/1994, 40162/1994, 199045/1994, 336086/1994, 76169/1995, 125441/1995, 262604/1995, 156218/1997, 193544/1997, 193545/1997, 193547/1997, 194748/1997, 202052/1997, 267562/1997 and 274732/1997. However, these conventional techniques have the problem of dye durability and particularly some problems peculiar to the uses of short wavelengths. For example, when a small pit is opened with a focussed laser beam, its periphery is widely affected, so that the largely expanded pit is inconveniently formed. This inconvenience causes the deterioration of a jitter and a radial cross talk. Conversely, in some of these conventional techniques, an extremely small pit is formed on occasion, and this inconvenience causes the deterioration of a modulated amplitude. In addition, when an organic dye having an unsuitable optical constant (a refractive index and an extinction coefficient) is selected and used for a recording layer at a desired laser wavelength, the deterioration of reflectance, sensitivity and the like occurs. Presently, these drawbacks have not been overcome at all yet.